Turbine components used in electrical power generation generally encounter severe wear due to a variety of mechanisms including: abrasion, erosion, fretting, corrosion, and metal-to-metal friction. Especially susceptible to this phenomenon are the keys and liners on the steam flow guides located on the upper and lower casing of a high pressure turbine. Such parts are typically manufactured from stainless steel, i.e., 12% Cr material. Since this metal is not hard in its tempered condition (15 to 25 Rockwell "C"), protective coatings such as hard facings or claddings are usually employed to prolong the life of these parts in service.
One such coating employed is STELLITE "6", which is an extremely hard material that has been a standard for hardsurfacing applications. It generally produces a surface which resists metal-to-metal wear, abrasion and impact. However, Stellite is often associated with cracking due, in part, to the differences between the coefficient of thermal expansion between the 12% Cr base metal and the welded deposit. Moreover, such cracking can also extend into the base metal, which often has been hardened intensely from the welding temperatures. This can lead to the premature failure of the component and necessitate its repair or replacement.
Replacement of these worn, fretted, or cracked components can be extremely costly. Down time alone can amount to $100,000 per day, since the electric utility often must buy electrical power elsewhere to meet consumer demands. In addition to this cost, the expenses associated with hiring a repair crew and purchasing and storing spare parts can be significant.
In an effort to reduce downtime and the consequent expense, new alloys are currently being developed to prolong the service life of turbine components. One such alloy is Tribaloy-400 from Cabot Corp of Kokomo, Ind. Tribaloy is a cobalt-based alloy and therefore retains its hardness even at elevated temperatures. See T. B. Jefferson, et al., Metals and How to Weld Them, James F. Lincoln Arc Welding Foundation, Cleveland, OH, February 1983, which is hereby incorporated by reference. Deposits of Tribaloy and the heat affected zone of the underlying 12% Cr stainless steel base metal, unfortunately, have developed cracks and pin holes after welding, and therefore, are not completely satisfactory.
For producing metallurgically sound weld deposits, the welding industry has traditionally relied on a "buttering" layer. Buttering has been disclosed in the trade literature as a means for applying a transition alloy to a base metal that will later be welded to a part of a different chemical composition. Birchfield, Part Worn or Undersized? Metal Overlays Save the Day, Welding Design and Fabrication, pp. 38-48, February 1985; which is hereby incorporated by reference. The Birchfield article reviews various processes and materials for the selection of metal overlaying. It discloses that buttering provides a metallurgical bridge between different alloys and that a buttering material must be readily weldable to the base metal and compatible with the joint filler metal that will unite the buttering part and mating part. Birchfield, also discloses the following examples: high-nickel weld metal deposited on a carbon or low-alloy steel substrate, to be welded later to a high-alloy steel base metal; a nickel-chromium-iron alloy deposited on a stainless-clad low-alloy steel before welding to stainless steel. Although teaching a use for a buttering layer, this reference fails to address the problems associated with the thermal shock and base metal cracking of turbine components. Moreover, this reference requires that the buttering layer be welded to a separate buttering part prior to attachment to a base metal mating part.
Accordingly, there is still a need for a method for hardsurfacing metal surfaces to provide turbine components having an extended useful life. There is also a need for a repair procedure that minimizes latent welding stresses and cracking of stainless steel, on power generation equipment.